The present invention relates to a lithium ion secondary battery and a carbon material for the negative electrode used in the lithium ion secondary battery.
As many more types of electronic equipment are being produced in portable versions and cordless versions in recent years, demands for secondary batteries, each having a small size, a light weight and a high energy density and serving as a power supply for the foregoing electronic equipment have been increasing.
In this regard, a nonaqueous electrolyte secondary battery, particularly a lithium ion secondary battery, has great expectations as a battery having a high voltage high energy density.
So far, a battery family employing transition metal oxides or sulfides such as manganese dioxides, molybdenum disulfides and the like to serve as a positive electrode and metallic lithium or alloys of lithium as a negative electrode has been put forth to produce lithium ion secondary batteries.
However, when metallic lithium is used as the negative electrode, the metallic lithium is deposited on the negative electrode in a needle-like shape or a moss-like configuration during a charge, piercing through separators and coming into contact with the positive electrode, thereby causing serious problems in safety of the battery such as a sudden rise in battery temperatures and the like due to internal short-circuiting.
As a result, use of a carbon material that can absorb and release lithium ions as the negative electrode has been proposed. In this case, lithium ions get into the carbon material between the layers thereof and no lithium is deposited on the negative electrode, thus eliminating the dangers of degrading the safety of the battery and at the same time contributing to an improvement in rapid charge characteristics. Because of these reasons, lots of RandD activities are being carried out at present in this particular area.
In these cases, a lithium-containing metal oxide such as LiCoO2, LiNiO2 or the like is used as the material for negative electrodes.
When such an accident as a battery being pressed strongly from both side surfaces occurs, however, the external pressure applied to the side surfaces of the battery used to cause the positive and negative electrodes to contact with each other after breaking through separators, resulting in development of an internal short circuit.
Upon developing the internal short circuit as described in the above, large currents flowing through the areas where the positive and negative electrodes are in contact with each other bring about a heat generation caused by Joule""s heat due to large contact resistance, thereby raising a problem of an abrupt increase in battery temperatures.
The present invention deals with the problem as described in the above and has an objective of providing much safer types of lithium ion secondary batteries, temperatures of which do not increase abruptly even if the batteries are crushed.
In order to accomplish this objective, the present invention has proposed a negative electrode for lithium ion secondary batteries that uses a material containing a carbon material of less than 5.0xc3x971031 3 ohmcm in volume resistivity, whereby absorption and release of lithium ions are made possible. Therefore, even when the batteries are crushed and internal short-circuiting takes place, generation of Joule""s heat is suppressed due to small volume resistance of carbon material.
Further, the present invention has disclosed a lithium ion secondary battery using a lithium-containing transition metal composite oxide as the positive electrode thereof and also using as the negative electrode thereof a carbon material that can absorb and release lithium ions and wherein volume resistivity is made less than 5.0xc3x9710xe2x88x923 ohmcm.
Furthermore, the present invention has disclosed use of a negative electrode wherein filling density of a carbon material ranges from 1.2 to 2.0 g/cc.